Advanced Placement (AP) Biology Practice Exam

Which type of cellular signaling involves direct contact between cells?

1. Paracrine signaling

2. Endocrine signaling

3. Direct signaling

4. Autocrine signaling

The correct answer is: Direct signaling

The type of cellular signaling that involves direct contact between cells is referred to as direct signaling. This process allows for communication between adjacent cells through mechanisms such as gap junctions or plasmodesmata in plant cells. In direct signaling, the signaling molecules are often passed directly from one cell to another, which is crucial for immediate responses or coordination of activities in a group of cells. In contrast to direct signaling, paracrine signaling involves the release of signaling molecules that affect nearby target cells, but without direct contact. Endocrine signaling refers to hormones released into the bloodstream that can affect distant cells, while autocrine signaling occurs when a cell releases signaling molecules that bind to receptors on its own surface or on cells of the same type, affecting only that cell or similar cells nearby. Each of these methods of signaling serves distinct functions in cellular communication, but direct signaling uniquely allows for immediate and localized interactions between cells.